1. Technical Field
The present invention relates to an amplifier circuit such as an inverting amplifier circuit, a variable resistor array used in the amplifier circuit and for adjusting the amplification degree (gain) and so on, and an electronic apparatus incorporating the amplifier circuit.
2. Related Art
In relation to a “switch for switching the amplitude of the gain of an inverted amplifier” described in JP-A-2007-258784, a variable resistor array RVAR in an amplifier circuit AMP shown in FIG. 4 is provided with a first NMOS transistor N1 and a second NMOS transistor N2 as shown in FIG. 5A in order for switching, for example, between a resistance value R1 of a resistor R1 and a combined resistance value (R1+R2) of a resistance value R1 of the resistor R1 and a resistance value R2 of a resistor R2.
The first and second NMOS transistors N1, N2 come into a conduction (ON) state or a cut-off (OFF) state exclusively, namely the second NMOS transistor N2 comes into the cut-off (OFF) state when the first NMOS transistor N1 is in the conduction (ON) state, and the second NMOS transistor N2 comes into the conduction (ON) state when the first NMOS transistor N1 is in the cut-off (OFF) state, for example.
As shown in FIG. 5B, the first NMOS transistor N1 can be represented in a equivalent manner with a first switch SW1 provided with an ON-resistor element Ron having a resistance value of Ron in the conduction (ON) state and an OFF-resistor element Roff having a resistance value of Roff in the cut-off (OFF) state. Similarly, as shown in FIG. 5B, the second NMOS transistor N2 can be represented in a equivalent manner with a second switch SW2 provided with the ON-resistor element Ron and the OFF-resistor element Roff.
In order for obtaining the former resistance value R1 with high accuracy, it is required that the relationship between the ON-resistance value Ron of the ON-resistor element Ron of the first switch SW1 in the conduction (ON) state of the first switch SW1 and the resistance value R1 of the resistor R1 satisfies R1>>Ron.
Further, in order for obtaining the latter combined resistance value (R1+R2) with high accuracy, it is required that the relationship between the combined resistance value (R1+R2) and the OFF-resistance value Roff of the OFF-resistor element Roff of the first switch SW1 satisfies Roff>>R1+R2 (>R1).
In summary of the two relational expressions described above, it is required to satisfy the relationship of Roff>>Ron. The relationship described above can also be obtained with respect to a variable resistor array RVAR shown in FIG. 6 obtained by generalizing the variable resistor array RVAR shown in FIGS. 5A and 5B, namely the variable resistor array RVAR composed of n resistors R1 through Rn and n switches SW1 through SWn.
However, in the related art, there is a problem that a complicated and sophisticated manufacturing process is required for manufacturing an MOS transistor with the ON-resistance value Ron (e.g., several hundreds of ohms) of the ON-resistor element Ron and the OFF-resistance value Roff (e.g., several hundreds of mega ohms) of the OFF-resistor element Roff, and in other words, there is a problem that the MOS transistors described above cannot easily be manufactured.